Field of the Invention
The invention relates to a process for structuring a photoresist layer.
In semiconductor technology, photolithographic methods play a key role in the production of integrated circuits on a semiconductor substrate. Typically, photoresist layers are applied to the surface of the substrate to be structured and are then structured by exposure to radiation from a suitable wavelength range. The exposure for structuring is effected by a lithography mask that predetermines the structure that is to be transferred to the substrate. The exposed parts of the photoresist layer are chemically modified by the exposure and their polarity is, thus, changed. For this reason, the exposed and the unexposed parts of the photoresist have different solubilities in a corresponding developer. This is used in the subsequent development step for selectively removing the exposed or unexposed parts. Those parts of the photoresist layer that remain on the substrate serve in the following structuring step as a mask that protects the substrate layer underneath from removal of material or modification of material. Such a structuring step may be, for example, plasma etching, wet chemical etching, or ion implantation.
Both in the case of the one-layer resists developable by a wet method and in the case of the two-layer resist systems developable completely or partly by a dry method, chemical amplification resists (CAR) have proven particularly useful as photoresists. Chemical amplification resists are characterized in that they contain a photoacid generator, i.e., a photosensitive compound that generates a protic acid on exposure to light. The protic acid then initiates acid-catalyzed reactions in the base polymer of the resist, if necessary with thermal treatment of the resist. As a result of the presence of the photoacid generator, the sensitivity of the photoresist is substantially increased compared with a conventional photoresist. An overview of this topic is given by H. Ito in Solid State Technology, July 1996, page 164 et seq.
In the case of the positive resists, the different solubilities of the exposed and of the unexposed photoresist is achieved by the principle of acid-catalyzed cleavage. A polar carboxyl group is formed thereby from a nonpolar chemical group of the layer-forming polymer, for example, a tert-butyl carboxylate group, in the presence of a photolytically produced acid, if necessary in a heating step. Further examples of nonpolar xe2x80x9cblockedxe2x80x9d groups that can be converted into corresponding polar groups by acid-catalyzed reactions are the tert-butoxycarbonyloxy (t-BOC) or acetal groups. Through the conversion of the nonpolar group into the corresponding polar group, the resist undergoes a change in polarity in the previously exposed parts, with the result that it becomes soluble in the polar, aqueous alkaline developer. Consequently, the developer selectively removes exposed parts of the photoresist. The resist residues in the unexposed, nonpolar parts, thus, geometrically define a resist profile or a resist structure on the substrate, which serves in subsequent process steps as a mask for surface structuring.
In contrast, a reduction in the solubility of the photoresist in the exposed parts is caused by the exposure in negative resists. To achieve such a result, negative-working photoresists have, as a rule, crosslinkable groups that can undergo crosslinking reactions under the influence of irradiation. As a result of the crosslinking, the solubility of the exposed parts of the photoresist in a corresponding developer is reduced. The crosslinkable groups can either be directly bonded to the base polymer or be present as a separate crosslinking component in the photoresist. In chemically amplified, negative-working photoresists, groups that are crosslinkable under acid catalysis and are activated by the photolytically liberated acid are used.
Due to the constantly increasing integration density in semiconductor technology, the accuracy with which the resist profile can be produced after development on a surface to be structured is of decisive importance. The resist profile is firstly physically uniquely predefined by the light distribution during the exposure. Secondly, it is chemically transferred to the resist layer by the distribution of the components produced photochemically by the exposure.
Due to the physicochemical properties of the resist materials, completely unfalsified transfer of the pattern predetermined by the lithography mask to the resist is, however, not definitively possible. Here, in particular, interference effects and light scattering in the photoresist play a major role. However, the steps following the exposure, such as, for example, the development, additionally have a considerable influence on the quality of the resist profiles, which is substantially determined by the profile sidewalls. To achieve surface structuring that is as precise as possible in the subsequent process steps, it would be ideal if it were possible to obtain virtually perpendicular, smooth profile sidewalls in the resist profile after development of the photoresist.
In particular, the light intensity profile that results in the photoresist during the exposure has an adverse effect on the steepness of the profile sidewalls that is to be achieved. This characteristic intensity profile, which is also referred to as xe2x80x9caerialxe2x80x9d image, is due to the light scattering and light absorption occurring in the resist during exposure. Because the photoresist absorbs a certain proportion of the incident radiation, the observed radiation intensity decreases with increasing layer thickness in the photoresist. Consequently, those parts of the photoresist layer that are close to the surface are more strongly exposed. In the case of a positive resist, the parts that are close to the surface are, thus, more highly soluble than the parts away from the surface. The different solubilities within an exposed part of the resist often leads to a flattening and to poor definition of the profile sidewalls in the case of positive resists. The light intensity profile in the photoresist thus describes the distribution of a photochemically modified species, for example, the distribution of the photolytically produced acid in the photoresist in the case of a positive resist.
In the case of negative resists, the decrease in the radiation intensity with increasing layer thickness in the photoresist leads to greater crosslinking in the parts that are close to the surface and that, thus, have a lower solubility than the parts away from the surface. In the subsequent development of the exposed photoresist, those parts of the photoresist layer that are away from the surface are, thus, dissolved to a greater extent than the parts on top that are close to the surface, with the result that the quality of the profile sidewalls and, hence, the resolution also deteriorate.
The quality and the steepness of the resist profiles are of decisive importance for the transfer of the structure from the photomask to the layer underneath that is to be structured. One prior art approach for improving the quality of resist profiles in positive resists, is described in European Patent Application EP 962825 A, corresponding to U.S. Pat. No. 2,012,867 A1 to Yasuda. There, an improved steepness of the resist sidewalls is achieved by adding to the photoresist two photochemically active additives that are activated by radiation in different wavelength ranges in each case. Firstly, the photoresist contains a photoacid generator that, as already described above, liberates an acid on exposure to light of a defined wavelength range, which acid then catalyzes the reaction of the convertible nonpolar groups of the layer-forming polymer of the photoresist to give carboxyl groups and, hence, causes the photoresist to be soluble in the polar developer. Secondly, the photoresist contains, as the second photochemical additive, a crosslinking reagent that results in a reduction of the solubility of the photoresist. The crosslinking reagent is likewise activated by radiation, the radiation used for such a purpose differing from the radiation used for activating the photoacid generator.
According to Yasuda, the photoacid generator is activated in the parts determined by the mask layout, in a first exposure step for structuring. In a subsequent, second floodlight exposure step, the total photoresist layer is exposed without the use of a photomask, and the crosslinking reagent is, thus, photochemically activated over the total surface of the photoresist layer. As a result of the chemical crosslinking of the photoresist that is thus initiated, the solubility thereof is reduced. Because those parts of the photoresist that are close to the surface are more strongly exposed, they are more strongly crosslinked and, hence, are more insoluble than the parts away from the surface. The change in the solubility is opposite to the change in solubility that is achieved in the first exposure step. While the exposed parts close to the surface have a higher solubility than the parts away from the surface as a result of the first exposure step, precisely the opposite gradient is produced by the second exposure step. Due to the selective solubility modification in the photoresist, increased developer selectivity in the aqueous developer is achieved, resulting in steeper resist profile sidewalls.
However, such an approach has a decisive disadvantage: the crosslinking reaction leads to the formation of a three-dimensional network polymer, particularly, in those parts of the photoresist that are close to the surface. The network polymer has altered development behavior compared with the original, linear layer-forming polymer, which leads to xe2x80x9croughxe2x80x9d, i.e., inexactly defined, e.g., frayed profile sidewalls. Such roughness complicates the subsequent process steps, such as, for example, the substrate etching.
It is accordingly an object of the invention to provide method for structuring a photoresist layer that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that reduces or completely avoids the disadvantages described above. In particular, the present invention provides a method by which highly accurate transfer of the structure predetermined by the lithography mask to a photoresist layer is achieved.